The present invention relates to a method of separating ZE-nepetalactone and EZ-nepetalactone from catnip oil involving mixing catnip oil dissolved in at least one water immiscible, non-halogenated organic solvent with at least one inorganic base dissolved in water to form a biphasic mixture, stirring the biphasic mixture to hydrolyze ZE-nepetalactone to form ZE-nepetalic acid, separating the aqueous phase containing ZE-nepetalic acid from the organic phase containing EZ-nepetalactone in the biphasic mixture, and optionally acidifying the aqueous phase to about pH 4.5 and adding at least one water immiscible, non-halogenated organic solvent to azeotropically lactonize the ZE-nepetalic acid in the presence of a catalytic amount of p-toluene sulfonic acid to form ZE-nepetalactone.
Many natural product components, including terpenes, alkaloids and esters, have potential to either attract or repel insects of broad spectrum. Plant species produce essential oils (aromatic oils) which are used as natural sources of insect repellents and attractants (Hay, R. K. M., Svoboda, K. P., Botany in Volatile Oil Crops: their biology, chemistry and production, Hay, R. K. M., Waterman, P. G. (Eds.), Longman Group UK Limited (1993)). Citronella oil, known for its general repellence towards insects, is obtained from the graminaceous plants Cymbopogon winterianus and C. nardus. Most of the plants yielding oil of value to the fragrance industry are members of the Labiatae (Lamiaceae) family. Plants of the genus Nepeta (catmints) are also members of this family and produce an essential oil which is a minor item of commerce. This catnip oil is very rich in a class of monoterpenoid compounds known as iridoids (Inouye, H., Methods in Plant Biochemistry, 7:99-143 (1991)), more specifically the methylcyclopentanoid nepetalactones and derivatives (Clark, L. J., et al., The Plant Journal, 11:1387-1393 (1997)).
Iridoid monoterpenes have long been known to be effective repellents and/or attractants to a variety of insect species (Eisner, T., Science, 146:1318-1320 (1964); Eisner, T., Science 148: 966-968 (1965); Peterson, C., and J. Coats, Pesticide Outlook, 12:154-158 (2001)). U.S. Pat. No. 4,663,346 disclosed insect repellants with compositions containing bicyclic iridoid lactones (e.g., iridomyrmecin). Further, U.S. Pat. No. 4,869,896 disclosed use of these bicyclic iridoid lactone compositions in potentiated insect repellent mixtures with DEET.
Two nepetalactone isomers exist in catnip oil from catmint, Nepeta cataria (Lamiaceae), and can be isolated by distillation (FIG. 1 shows the chemical structures of the naturally-occurring iridoid (methylcyclopentanoid) nepetalactones). One isomer, (4aS, 7S, 7aR)-nepetalactone (cis-trans nepetalactone or ZE-nepetalactone), and its alcohol form (1R, 4aS, 7S, 7aR)-nepetalactol, have been identified as sex pheromones for numerous species of aphids. The isomer (4aS, 7S, 7aS)-nepetalactone (trans-cis nepetalactone or EZ-nepetalactone) is reportedly useful as a cockroach and mosquito repellent (Dawson, G. W., et al., Bioorganic & Medicinal Chemistry, 4(3): 351-361 (1996); Peterson, C. J., and Coats, J., Catnip repels mosquitoes more effectively than DEET, American Chemical Society, 222nd National Meeting (2001); Coats, J., Catnip and Osage orange components found to repel German cockroaches, American Chemical Society, 218th National Meeting (1999)). Since catnip oil has exhibited promising repellent and toxic effects against subterranean termites, nepetalctone isomers are also being evaluated as termiticides (Peterson, C. J., and Elm-Wilson, J., J. Econo. Entomology, 96 (4): 1275-1282 (2003)).
The isomer (4aS, 7S, 7aR)-nepetalactone has been commercially produced in 85-97.5% enantiomeric purity from one selected cultivar of catmint (Birkett, M. A., and J. A. Pickett, Phytochemistry, 62: 651-656 (2003)); this genetically different cultivar of Nepeta cataria containing (4aS, 7S, 7aR)-nepetalactone had to be planted and oil obtained from this particular species was used to isolate the ZE-isomer, and therefore is not cost effective. Availability of the other isomer, (4aS, 7S, 7aS)-nepetalactone, still relies on chromatographic separation or chemical synthesis. These traditional methods, chromatographic separation or chemical synthesis of two pure nepetalactone isomers, are labor intensive and expensive. Since both isomers are ecologically important and available from non-specific catnip oil at low price, there is a need for a method that could effectively separate diastereomers of nepetalactones on a large scale and at a low cost.